minor symfony#22367 [Yaml] release memory after parsing (xabbuh)

This PR was merged into the 2.7 branch.

Discussion
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[Yaml] release memory after parsing

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#22351 (comment)
| License       | MIT
| Doc PR        |

Follow up of symfony#22351 to release memory after parsing as suggested by @stof.

Commits
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c891413 [Yaml] release memory after parsing

Author: fabpot

src/Symfony/Component/Yaml/Parser.php

@@ -86,6 +86,12 @@ public function parse($value, $exceptionOnInvalidType = false, $objectSupport =
             mb_internal_encoding($mbEncoding);
         }
 
+        $this->lines = array();
+        $this->currentLine = '';
+        $this->refs = array();
+        $this->skippedLineNumbers = array();
+        $this->locallySkippedLineNumbers = array();
+
         if (null !== $e) {
             throw $e;
         }